Transmission of digital information on a physical device such as on wires on a computer chip, through optical cables, through twisted pair copper wires, or through cables such as the High-Definition Multimedia Interface (HDMI), and other such physical, tangible and/or non-transitory transmission media, has been the subject of much investigation. Each piece of the information to be transmitted may be associated with a continuous time waveform in such a way that different pieces of information are distinguishable from one another by means of their corresponding waveform. For example, where information is transmitted on wires using physical voltages, the pieces of information could correspond to the possible values 0 or 1 of a bit, and a 0 could correspond to a voltage of +V with respect to fixed reference voltage, whereas a 1 could correspond to a voltage of −V with respect to the same reference voltage. Where higher modulation is used, the pieces of information could correspond to all possible combinations of a larger number of bits, and each such group of bits could correspond to a different phase of the waveform, or to a different amplitude, or to a different frequency. Information may be encoded into a waveform by altering one or more of its properties. This waveform may be converted into a physical, tangible and/or non-transitory embodiment that can be carried by the transmission medium. The process of varying one or more properties of a waveform with respect to a baseline modulating signal is herein called the process of modulation. The act of transmitting the information using a modulated signal is herein referred to as signaling.
One of the more commonly used methods for transmitting signals over wires is the single-ended signaling method. Where information is transmitted by either applying different voltage levels (voltage mode) on the wire with respect to a reference or sending a current (current mode) with different strengths into the wire. When using voltages, one wire carries a varying voltage that represents the signal while the other wire is connected to a reference voltage which is usually the ground. When using currents, the common return path of the current is usually ground. Hence, in a single-ended signaling method each signal source is connected to the data acquisition interface using one signal path. At the data acquisition interface, often voltages are measured, either, directly or by terminating the wire by means of a load to a reference. The result of the measurement is proportional to the difference between the signal and the reference, often “ground” or “earth”, at the acquisition point. The method relies on the signal source reference to be the same as the data acquisition point's reference. However, in reality they can be different for a variety of physical reasons. This is especially problematic when signals have to traverse longer distances (for example in twisted pair copper wires), or when the frequencies of the signals are very high (for example in high throughput on-chip communication). Using ground as a reference and connecting the grounds on both ends can drive large currents known as ground loops, which can lead to significant errors when using single-ended inputs. Furthermore, it is not always possible to have a common electrical reference at both sides of the wires. This can be the case in optical communications. Moreover, single-ended inputs can be susceptible to noise (i.e., unwanted signal contaminations). For example, such noise can be added because electrical signal wires act as aerials, and hence pick up environmental electrical activity. Single-ended signaling methods do not always provide sufficient protection against these sources of noise, especially for high speed communications.
To combat these problems, a different form of signaling called differential signaling is used. In conventional differential signaling, information is transmitted using two complementary signals sent on two separate wires, for example, in the form of a voltage difference between the wires or current strength and direction in the wires. The output signal is then the difference between these two complementary signals. The technique can be used for both analog signaling, for example in some audio systems, and in digital signaling. Examples include, but are not limited to standards such as the RS-422, RS-485, the twisted-pair Ethernet, the Peripheral Component Interconnect (PCI) Express, the Universal Serial Bus (USB), serial ATA, Transition Minimized Differential Signaling (TMDS) used in DVI and HDMI cables, or the FireWire interface. While sending complementary signals on the two wires of a differential input is advantageous in some applications, it is not strictly required. Instead, it is possible to send the information on the second wire in such a way that the difference in voltage between the second and the first wire is +V or −V for some fixed voltage V. At the acquisition point, the receiving device reads the difference between the two signals. Since the receiver ignores the wires' voltages with respect to ground, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal. One of the main advantages of differential signaling is its resistance to “common-mode noise.” This is noise that affects both wires in the system in the same way (for example through interference caused by nearby wires). Furthermore, compared to single-ended signaling, the signal swing at the receiver is typically larger which can result in better noise performance.
In many practical scenarios, communication takes place by sending more than one bit of information a time. For example, in a 32-bit bus system, 32 bits of binary information may be sent simultaneously. In such cases, multiple single-ended, or differential, signaling paths are used in parallel, one for each bit. A disadvantage of conventional differential signaling in these practical scenarios is the large number of wires needed: to each bit there are two wires associated. The ratio between the number of bits and the number of wires used to transmit these bits is herein called the pin-efficiency of the system. This number is 1.0 (or 100%) for single-ended signaling, whereas it is 0.5 (or 50%) for differential signaling. In many communication scenarios, this makes the use of differential signaling less desirable. To combat this problem, in some cases the signals are serialized and sent over only one, or only a few pairs of differential signal paths. However, this method has the disadvantage of requiring the transmission to take place at a higher frequency in order to maintain a given throughput. However, transmission at higher frequency requires more energy both in order to realize the higher frequency of operation, and to combat noise associated with this mode of operation.
What is needed is a signaling method that, at least, retains the resilience of differential signaling against various modes of noise, and has a pin-efficiency that can approach that of single-ended signaling. Embodiments of the invention are directed toward solving these and other problems individually and collectively.